This invention relates to a trigger switch assembly, and particularly, to such a switch adapted for portable electric powered hand tools such as drills and the like.
Many hand power tools used in the home and industry feature variable speed control devices which are modulated by a trigger switch. These control devices permit the user to select the desired operating speed of the tool. For example, a drill equipped with a variable speed control enables the user to control the speed in accordance with the diameter of the drill bit being used or the workpiece material. Variable speed control systems further enable a portable power tool such as a drill to be used for various applications; for example, as a power screwdriver, sander, nut driver, etc. Typical variable speed control devices used in portable power tools are controlled by a trigger switch which causes the tool to operate at increasing power settings as the trigger is retracted. As an additional feature, many currently available trigger switches further incorporate a locking mechanism which enables the tool to operate at a selected power setting without continued application of pressure on the trigger switch. Certain of these devices further provide a lock adjuster mechanism which permits the power setting set by the locking device to be varied as desired.
This invention relates to an improved variable speed trigger switch of the above-described type having a number of improvements over those currently available. The trigger switch according to the present invention is uniquely configured to permit inexpensive, rapid automated assembly of the unit. More particularly, the trigger switch in the preferred embodiment is comprised of a clam-shell type housing which, when assembled, captures a miniature printed circuit board therebetween which contains the electronic motor speed control circuit (except for certain power supply components) on one side thereof and the variable resistance strip and stationary ON/OFF contacts of the switch on the other side. Additionally, electrical contacts for the line, motor and a tachometer generator are formed by conductive pads directly on the circuit board. In this manner, the overall size of the trigger switch is kept to a minimum, despite the inclusion of a sophisticated integrated circuit-based motor speed control circuit which allows the present trigger switch to be substituted in a wide variety of power tool applications without requiring the redesign and/or enlargement of present tool handle configurations.
Many present designs of variable speed trigger switches include locking mechanisms which have the disadvantage that, when a locked position less than a full power setting is selected, the range of travel of the trigger becomes limited by the locking mechanism even when the trigger is not locked. If the user of such a device desires full power output, the locking mechanism must be adjusted to permit full trigger travel even though the user does not intend to use the locking mechanism. Such designs further have the disadvantage that, when the locking device is being used, the user cannot further depress the trigger for an instantaneous increase in power without first readjusting the lock adjusting mechanism.
A variable speed trigger switch according to this invention has a locking mechanism which can be infinitely adjusted but which does not interfere with full travel of the trigger switch such that full power operation is always available to the user. As a further improvement over present variable speed trigger switches, the switch according to this invention further provides a detent which is felt by the user as a slight increase in resistance at a particular point as the trigger is depressed. The trigger position at which the detent is felt is adjustable in accordance with this invention through actuation of the lock adjusting mechanism. The detent enables the user to operate the tool at a desired power setting without engaging the locking mechanism if so desired. In a preferred embodiment of this invention, the adjusted lock position and the detent trigger position are the same. This configuration enables the detent to function as a means of enabling the user to quickly locate the trigger position at which the trigger locking mechanism can be engaged. The trigger switch in accordance with this invention further is well suited for portable power tools since it is lightweight, compact, inexpensive, and is suited for mass production. Further, the present switch features excellent heat dissipation characteristics, thereby providing electrical performance advantages. The preferred embodiment of the present invention also includes a novel electronic motor speed control circuit which provides a combination of open loop and "quasi" closed loop motor speed control. In particular, during trigger switch setting below a predetermined setting, the control circuit will operate in an open loop control fashion, whereas when the trigger switch is retracted beyond the predetermined trigger setting, the control circuit provides a varying degree of closed loop control.